Previous interpretations of the kinetics of transport of hydrophobic ions through membranes have been based on one of three limiting assumptions. Either diffusion in the aqueous phase was taken to be rapid, or ionic motion was constrained to the membrane or a steady state was presumed to be established within the membrane. We present a general treatment of the coupled diffusion process through both the aqueous phase and the membrane; our theory contains the previous results as limiting cases. It is applied to voltage jump-current relaxation experiments on black lipid membranes in the presence of dipicrylamine or sodium tetraphenylborate. We have attempted to establish the rate of desorption from the membrane. For the system phosphatidylserine/tetraphenylborate, the rate of desorption and the rate of translocation were found to be comparable.